A person experiencing chest pain has gone to the emergency room (ER). Generally, chest pain may be a symptom of a number of conditions, including cardiac and non-cardiac in origin. Often times, the person has no history of cardiac disease and the origin of the chest pain is unknown. A typical approach to diagnosing the person is to rule out certain causes of chest pain (differential diagnosis) such as myocardial infarction (MI or heart attack), pulmonary embolism, thoracic aortic dissection, etc. A standard procedure of the ER is to test troponin (a protein of muscles) levels of the person, perform an electrocardiogram (ECG) to obtain signals representative of the electrical activity of the heart, and/or admit the person for observation. When the results of such procedures are inconclusive, the person has further undergone invasive coronary angiography, stress testing, surgery, and/or other non-invasive imaging (e.g., a chest x-ray) to facilitate determining the cause of the chest pain.
Computed tomography (CT) is a non-invasive imaging procedure. A conventional CT scanner generally includes an x-ray tube mounted on a rotatable gantry opposite a detector array across an examination region. The rotatable gantry, and hence the x-ray tube, can be rotated, under system control, around the examination region. The rotatable gantry, and hence the x-ray tube, can also be parked or held at a static angular position with respect to the examination region. The x-ray tube is configured to emit radiation that traverses the examination region and is detected by the detector array. The detector array, in response, generates and outputs a signal indicative of the detected radiation. The signal is reconstructed to generate a two dimensional projection image (e.g., a scout/pilot image) or three dimensional volumetric image data, depending on the type of scan performed.
The resulting projection image or volumetric image data includes pixels or voxels that typically are represented in terms of gray scale values corresponding to relative radiodensity. The gray scale values reflect the attenuation characteristics of the scanned subject and/or object, and generally show structure such as anatomical structures within the scanned patient or object. Since the absorption of a photon by a material is dependent on the energy of the photon traversing the material, the detected radiation also includes spectral information, which provides additional information indicative of the elemental or material composition (e.g., atomic number) of the scanned material of the subject and/or object. Unfortunately, conventional CT data does not reflect the spectral characteristics as the signal output by the detector array is proportional to the energy fluence integrated over the energy spectrum.
A spectral CT scanner captures the above-noted spectral characteristics. Generally, a spectral CT scanner may include two or more x-ray tubes configured to emit radiation having different mean spectrums, a single x-ray tube configured to be controllably switch between at least two different emission voltages (e.g., 80 kVp and 140 kVp) during scanning, and/or a single broad spectrum x-ray tube and an energy-resolving detector array with energy-resolving detectors (e.g., with photon counting detectors, at least two sets of photodiodes with different spectral sensitivities, etc.) and discrimination electronics. K-edge spectral imaging leverages the phenomena that high-Z elements tend to attenuate photons to a much higher extent above a particular energy (the K-edge energy of the given element) relative to attenuating photons just below the K-edge energy. The discontinuity in the attenuation behavior can be detected using an energy-resolving detector.
Unfortunately, CT scanners emit ionizing radiation and thus expose the person being scanned to ionizing radiation, which may damage or kill cells and/or increase the risk of cancer. As such CT scans have not become part of the clinical standard for non-invasive differentiation of chest pain. Thus, there is an unresolved need for lowering dose so that CT scanners can become part of the clinical standard for non-invasive differentiation of chest pain and/or other procedures.